Electrochemical batteries are a principal means for storing and delivering electrical energy. Due to increasing demands for energy for electronic, transportation and grid-storage applications, the need for batteries with ever more power storage and delivery capability will continue long into the future.
Because of their light weight and high energy storage capacity as compared to other types of batteries, lithium ion batteries have been widely used since the early 1990's for portable electronic applications. However, current Li-ion battery technology does not meet the high power and energy needs for large applications such as grid storage or electric vehicles with driving ranges that are competitive with vehicles powered by internal combustion engines. Thus, extensive efforts in the scientific and technical communities continue to identify batteries with higher energy density and capacity.
Sodium-sulfur and lithium-sulfur electrochemical cells offer even higher theoretical energy capacity than Li-ion cells and thus have continued to elicit interest as “next-generation” battery systems. Electrochemical conversion of elemental sulfur to the monomeric sulfide (S2−) offers a theoretical capacity of 1675 mAh/g as compared to less than 300 mAh/g for Li-ion cells.
Sodium-sulfur batteries have been developed and launched as commercial systems. Unfortunately, the sodium-sulfur cell typically requires high temperatures (above 300° C.) to be functional, and thus is only suitable for large stationary applications.
Lithium-sulfur electrochemical cells, initially proposed in the late 1950's and 1960's, are only now being developed as commercial battery systems. These cells offer theoretical specific energy densities above 2500 Wh/kg (2800 Wh/L) vs. 624 Wh/g for lithium ion. The demonstrated specific energy densities for Li—S cells are in the range of 250-350 Wh/kg, as compared to 100 Wh/g for Li-ion cells, the lower values being the result of specific features of the electrochemical processes for these systems during charge and discharge. Given that the practical specific energies of lithium batteries are typically 25-35% of the theoretical value, the optimum practical specific energy for a Li—S system would be around 780 Wh/g (30% theoretical). [V.S. Kolosnitsyn, E. Karaseva, U.S. Patent Application 2008/0100624 A1]
The lithium-sulfur chemistry offers a number of technical challenges that have hindered the development of these electrochemical cells, particularly poor discharge-charge cyclability. Nonetheless, because of the inherent low weight, low cost, high power capacity of the lithium-sulfur cell, great interest exists in improving the performance of the lithium-sulfur system and extensive work has been performed in the last 20 years by many researchers all over the world to address these issues. [C. Liang, et al. in Handbook of Battery Materials 2nd Ed., Chapter 14, pp. 811-840 (2011); V. S. Kolosnitsyn, et al., J. Power Sources 2011, 196, 1478-82; and references therein.]
A cell design for a lithium-sulfur system typically includes:                An anode consisting of lithium metal, lithium-alloy or lithium-containing composite materials.        A non-reactive but porous separator between the anode and cathode (often polypropylene or α-alumina). The presence of this separator results in separate anolyte and catholyte compartments.        A porous sulfur-bearing cathode that incorporates a binder (often polyvinylidene difluoride) and a conductivity-enhancing material (often graphite, mesoporous graphite, multiwall carbon nanotubes, graphene),        An electrolyte consisting of a polar aprotic solvent and one or more conductive Li salts [(CF3SO2)2N−, CF3SO3−, CH3SO3−, ClO4−, PF6−, AsF6−, halogens, etc.]. The solvents used in these cells have included basic (cation-complexing) aprotic polar solvents such as sulfolane, dimethyl sulfoxide, dimethylacetamide, tetramethyl urea, N-methyl pyrrolidinone, tetraethyl sulfamide, tetrahydrofuran, methyl-THF, 1,3-dioxolane, diglyme, and tetraglyme. Lower polarity solvents are not suitable due to poor conductivity and poor ability to solvate Li+ species, and protic solvents can react with Li metal. In solid-state versions of the lithium-sulfur cell, the liquid solvents are replaced with a polymeric material such as polyethylene oxide.        Current collectors and appropriate casing materials.        